Okay, then the problem isn't a ground loop, because that would also affect the other speakers, and the problem isn't reversed polarity because the three-prong plugs can only be connected with the correct polarity. If you're using a reasonably well-shielded coaxial cable to the sub, that shouldn't be picking up significant hum unless you have it right alongside a power cord(move the cable around to see if it makes any difference). Checking with Axiom might get some useful suggestions.

-----------------------------------

Enjoy the music, not the equipment.